As the widespread use of the Internet and mobile phones has increased communication capacities in recent years, backbone optical communication systems are required to have larger capacities, and research and development have been carried out for optical transmitter-receivers having a communication capacity of 40 Gbit/s, 100 Gbit/s, or higher for a single wavelength.
However, if a transmission capacity per single wavelength is increased, the quality of transmitted signals deteriorates greatly due to a lowered OSNR (Optical Signal to Noise Ratio), waveform distortions caused by wavelength dispersion on transmission paths, polarization mode dispersion, nonlinear effects, and the like.
Accordingly, digital coherent receiving schemes resistant to poor OSNR and also resistant to waveform distortions on transmission paths are gathering attention as schemes for optical communication systems yielding 40 Gbit/s or higher.
According to conventional receiving schemes, ON and OFF signals based on light intensities are assigned to binary signals to be used for direct detection (OOK: On-OFF Keying). By contrast, according to digital coherent receiving schemes, light intensity and phase information are extracted using a coherent receiving system, and the extracted light intensity and phase information are quantized using an analog/digital converter (ADC), and thereby demodulation is performed by a digital signal processing circuit.
Digital coherent receiving schemes are capable of improving the resistance to poor OSNR by using coherent receiving schemes, and are capable of compensating for waveform distortions by using a digital signal processing circuit, and accordingly are capable of suppressing deteriorations of quality of transmitted signals even when a communication capacity for one wavelength is large. Also, wavelength distortions may be compensated for by a digital signal processing circuit, which enables relatively flexible responses to transmission route modifications caused by network configuration modifications.
Further, digital coherent receiving schemes may be combined with modulation schemes capable of transmitting multi-bit information for one symbol so as to construct transmission systems yielding high-frequency efficiencies. As modulation schemes of this type, multivalued modulation schemes such as QPSK (quadri-phase shift keying), 8PSK, 16QAM, and 256QAM that multiply phase information and intensity information, a polarized multiplexing scheme that multiplexes different information onto orthogonal polarized waves, and a multi-carrier multiplexing scheme that multiplexes different information onto a plurality of frequencies that have been multiplexed highly densely within one wavelength grid (sub carriers), etc., are known. Among multi-carrier multiplexing schemes, OFDM (Orthogonal Frequency Division Multiplexing), in particular, is considered to be promising as a future optical communication method.
Generating an OFDM signal requires a plurality of beams of light whose frequencies have been synchronized. In addition, the frequency and the wavelength of light correspond to each other in a one-to-one manner, and they act as carriers, and accordingly, they are referred to as the frequency of light, the wavelength of light, or the carrier in the explanations below.
Further, it is desirable that the number of carriers of OFDM (i.e., the total bit rate per one OFDM signal) be variable in response to the traffic of a network.
FIGS. 1A and 1B illustrate an example of a conventional multi-wavelength light source.
In the multi-wavelength light source illustrated in FIG. 1A, continuous light with a center wavelength of f0 is input, from a laser diode 15, to an optical circulation unit including an optical SSB (Single Side-Band) modulation device 10, optical amplifiers 11 and 12, an optical filter 13, an optical multiplexer (optical coupler) 14, and an optical demultiplexer (optical coupler) 16. Periodic waves with frequency Δf as a drive signal and periodic waves that are phase-shifted by π/2 from the first periodic waves are input to the optical SSB modulator in the optical SSB modulation device 10.
Input light passes through the optical SSB modulation device 10, and thereby has its frequency shifted by Δf to the higher side so that f1=f0+Δf. Part of the light is output (zero-circulation output in FIG. 1B), and part of the remaining light propagates in the optical circulation unit, passes through the optical filter 13 and the optical amplifier, and is again input to the optical SSB modulation device 10.
The continuous light with its center frequency f1 passes through the optical SSB modulator in the optical SSB modulation device 10, and thereby has its center frequency shifted by Δf to center frequency f2. Similarly, part of the light is output (one-circulation output in FIG. 1B), and part of the remaining light propagates in the optical circulation unit. By repeating this, multi-wavelength continuous light having f1 through f6 may be obtained through five-circulation output. In the example in FIG. 1A, light having f2 or higher will not be generated because the optical filter 13 in the optical circulation unit is set to have a transmission bandwidth that transmits from f1 through f5.
As has been described above, the optical SSB modulation device 10 shifts the frequencies of input continuous light, and thus the optical SSB modulation device 10 is also referred to as an optical frequency shifter. An optical frequency shifter is not limited to a device that uses an optical SSB modulator if the shifter is capable of shifting frequencies of input continuous light.
In a multi-wavelength light source including the optical SSB modulation device 10 and an optical circulation unit as described above, the OSNR (Optical Signal to Noise Ratio) of a wavelength with a large number of times of circulation deteriorates when such a wavelength light source generates a large number of wavelengths.
FIG. 2 explains OSNR deterioration.
FIG. 2 is a graph having the horizontal axis representing the number of generated carriers and the vertical axis representing the OSNR of each carrier to depict how the OSNR of each carrier changes in response to an increase in the number of generated carriers. When the number of generated carriers is one, the OSNR of the carrier is 50 dB or higher, while when the number of generated carriers has reached eight, the OSNR of each carrier drops to 40 dB. Further, when the number of generated carriers increases to nine or ten, the OSNR of each carrier drops further.
FIG. 3 illustrates a configuration that has conventionally been required when a plurality of wavelengths are to be used for communication.
As illustrated also in FIG. 2, a single conventional multi-wavelength light source is capable of generating at most about eight wavelengths due to the OSNR of each carrier. Accordingly, as illustrated in FIG. 3, an optical communication system that multiplexes several tens of waves or several hundreds of waves needs to be provided with many multi-wavelength light sources such as those illustrated in FIG. 1A. However, this configuration increases the number of light sources and also increases the cost of the communication system.
Also, as conventional techniques, there are a fiber-ring-resonator-type light source that oscillates multi wavelengths simultaneously, a wavelength management device for securing wavelength stability of a multi-wavelength light source, a multi-wavelength light source that is capable of simultaneously providing a multi-wavelength continuous light having constant intervals, and the like.    Patent Document 1: Japanese Laid-open Patent Publication No. 10-93164    Patent Document 2: Japanese Laid-open Patent Publication No. 2000-47278    Patent Document 3: Japanese Laid-open Patent Publication No. 2005-77584